The invention relates to vehicle automatic transmissions with multi-stage gear ratios. More precisely, it relates to a process for control of downshifts of the gear ratios, which process permits improvement of vehicle driving comfort and safety, by acting on the braking or acceleration capacity of an automatic transmission vehicle.
In most methods for control of gear ratio changes in automatic mode, the gear ratio changes are authorized as a function of vehicle speed and engine load. Most often, these criteria are translated in the form of gear ratio change laws, which are conveniently visualized in a Vveh/xcex1pap graph, where Vveh denotes the vehicle speed and xcex1pap the throttle opening angle, or more generally the opening of the device for regulation of admission of fuel to the engine.
A known example of gear ratio change laws is proposed in FIG. 1, in which curves 1 and 3 respectively represent the curves of downshift from N to Nxe2x88x921 and from Nxe2x88x921 to Nxe2x88x922, and curve 2 represents the curve of upshift from Nxe2x88x921 to N, where N is the current gear ratio.
As will become apparent hereinafter, the known processes for control of gear ratio changes of automatic gearboxes with multi-stage gear ratios exhibit disadvantages when it is desired to reduce the gear ratios, both in the case of braking and in that of acceleration.
The problem of braking assistance during downshifts will be discussed first of all with reference to FIGS. 1 to 3, and especially to FIG. 1. Let us suppose that the driver is traveling at stabilized speed in fourth gear (assuming that N=4) and that, for some reason, he is forced to slow his vehicle substantially by braking. Downshifts will of course be executed, but in the following manner: starting from point A (speed stabilized in gear ratio N=4), the driver will progressively release the accelerator in order to then brake. He will then pass from A to B, still in the same gear ratio N=4. Since he is braking, the vehicle speed will drop and the current point will successively intersect downshift curves 1 and 3 at points C and D. The downshift from N to Nxe2x88x921 takes place at C and the second downshift from Nxe2x88x921 to Nxe2x88x922 (which in the example chosen is from third to second gear) takes place at D.
This analysis shows clearly that the downshifts will invariably take place at points C and D, regardless of braking intensity, regardless of traveling conditions (downhill, level) and regardless of the driver""s driving style. As it happens, this situation is not that desired by the driver, as is clearly shown by the driving manner of a driver who decides to change gear ratio in a manual gearbox vehicle, where the gear ratio changes may be made under highly variable conditions. Of course, the driver driving a vehicle equipped with an automatic gearbox still has the option of downshifting as desired by acting on the gearstick, but in this case the xe2x80x9cautomaticxe2x80x9d performance is lost.
A first objective of the present invention is therefore to provide a process for assisting braking by anticipatory downshifts of automatic transmissions with multi-stage gear ratios, which process can adapt the gear ratio change decisions in the phases of braking of the vehicle to a plurality of parameters and is capable of eliminating the disadvantages of the prior art.
Another objective of the invention related to the problem of braking assistance is to achieve supplementary engine braking of the vehicle, to downshift to a xe2x80x9cgoodxe2x80x9d gear ratio on entering a curve, and on the whole to add more driving comfort during vehicle deceleration phases.
According to another aspect, the invention relates to a process for control of downshift phases of automatic transmissions with multi-stage gear ratios, which process makes it possible to assure driving safety in the acceleration phase, for example while the vehicle is in passing situations or during traveling situations which necessitate a potential for hard acceleration.
This problem will be explained in greater detail with reference to FIG. 4. In this figure there is proposed an example of a gear ratio change law in which curves 4 and 5 respectively represent the curves of downshift from N to Nxe2x88x921 and from Nxe2x88x921 to Nxe2x88x922, where N is the current gear ratio of the transmission.
Let us suppose that the driver is traveling at stabilized speed in fourth gear (assuming that N=4), which therefore corresponds to point A in the xcex1pap, Vveh graph described hereinabove. If for some reason of his own the driver wishes to accelerate strongly (to pass another vehicle, for example), he needs all available power to the wheels in order to achieve the maximum acceleration potential. Referring to FIG. 4, upon opening of the throttle, the current point in the upap, Vveh graph will therefore travel along the segment oriented from A to D, successively intersecting the downshift curves 4 and 5. Gear ratio Nxe2x88x921 will be engaged at point B, and gear ratio Nxe2x88x922 will then be engaged by the transmission at point C. A sequence from N to Nxe2x88x921 and from Nxe2x88x921 to Nxe2x88x922 is therefore observed in the cascade of gear ratios. This sequence is not desired from the safety viewpoint, because it is well known for delaying the appearance of maximum power at the wheel: the time for executing the two gear ratio changes may be too long, and may last several seconds depending on the transmission.
Under this second aspect, an objective of the invention is therefore to provide a process for control of downshift phases, which process makes it possible, during certain actuations of the accelerator, to avoid the cascades of N to Nxe2x88x921 and from Nxe2x88x921 to Nxe2x88x922 which detract from vehicle safety, in such a way that downshifting from N to Nxe2x88x922 (skipping a gear ratio) is executed directly, thus instantaneously providing the maximum power to the motive wheels.
To solve the problems posed by downshifts under braking or acceleration conditions as explained hereinabove, the invention provides corresponding process for control of downshifts.
For control of braking events, the invention therefore relates to
a process for control of downshifts of automatic transmissions, characterized in that it comprises:
determining the conditions under which the downshift shifts from normal operation as a function of predetermined shift laws to a special downshift mode, and detecting the shift to the said special mode;
in the event of detection of the shift to the special mode, downshifting by shifting from the current gear ratio (N) to a lower gear ratio, and disabling this lower gear ratio until disappearance of the special downshift mode.
According to other characteristics of the process, it is provided with the stages comprising:
a) to determine the conditions under which downshifting no longer takes place according to the normal automatic shift laws of the gearbox, but according to a special mode there is used a limit value (N_turb_max) for maximum speed of the torque converter turbine, below which the said special mode begins;
b) When during braking the turbine speed drops below the said limit value, an anticipatory downshift of one gear ratio is executed to shift from gear ratio N to gear ratio Nxe2x88x921, such that the anticipatory downshift participates in the braking effect.
preferably the limit value of turbine speed is determined in fuzzy logic, taking into account the vehicle deceleration representative of the braking intensity, the braking duration, a variable representative of the driver""s driving style, and a variable representative of the highway gradient.
In addition, the distance traveled by the vehicle is calculated as soon as the vehicle deceleration reaches a certain predetermined level and, to exit from the special downshift mode, the current gear ratio is enabled either when the vehicle has traveled a certain predetermined distance with the opening (xcex1pap) of the device for regulation of admission of fuel to the engine having remained stable, or when the engine rpm becomes too high, or when the N/Nxe2x88x921 downshift curve is intersected, or even when the driver manually actuates the gearstick to select a higher gear ratio.
Secondly, in order to solve the problems posed by downshifts for the purposes of vehicle acceleration, the invention relates to a process for control of downshifts according to one of the preceding claims, and used for purposes of acceleration assistance, characterized in that it is provided with stages comprising:
a) at the intersection with a downshift curve, starting a first time delay (Ta) of short duration, during which the downshift is inhibited and the current gear ratio N is retained;
b) during the said first time delay (Ta), analyzing the stability of the accelerator or of the throttle angle (xcex1pap) as a function of time;
c) in the case of stability above a predetermined threshold, selecting the gear ratio which corresponds to the current point in the graph (xcex1pap, vehicle speed), otherwise starting a second time delay (Tunstable), during which the gear ratio N is still retained;
preferably, the second time delay (Tunstable) is maintained and the gear ratio N retained as long as the instability (|xcex94xcex1pap|) lies between predetermined low and high thresholds of instability
when the second time delay (Tunstable) has elapsed, the gear ratio corresponding to the current point in the cartographic graph (xcex1pap, Vveh) of gear ratio changes according to normal operation is selected.
preferably the disabling for instability of the throttle opening angle has priority over disabling resulting from the waiting time delay.
The invention also relates to an automatic transmission using the above process for control of gear ratio downshifts, as well as to a motor vehicle with automatic gearbox having multi-stage gear ratios, provided with such an automatic transmission.
The disadvantages of current downshift processes will be explained in more detail with reference to FIG. 4. If the driver while in fourth gear, for example, rapidly depresses the accelerator starting from point A, he will intersect the downshift curve 4. But since he has depressed the accelerator rapidly, downshifting is disabled and a time delay Tunstable is started, and it will remain initialized as long as instability of xcex1pap is detected. Thus, when the current point stabilizes at point D, the gear ratio Nxe2x88x922 will be engaged and, by virtue of the invention, the sequence of N to Nxe2x88x921 and Nxe2x88x921 to Nxe2x88x922 will be avoided, thus ensuring safety during passing maneuvers as well as lively response of the transmission.
The invention also relates to an automatic transmission with multi-stage gear ratios using the process having the characteristics described hereinabove for control of downshifts, as well as to a vehicle incorporating such an automatic transmission.